utils: Add DnD target widget

Another utility function.

src/utils/gcal-gui-utils.c

@@ -30,3 +30,11 @@ gcal_create_selection_widget (void)
                        "css-name", "selection",
                        NULL);
 }
+
+GtkWidget*
+gcal_create_drop_target_widget (void)
+{
+  return g_object_new (ADW_TYPE_BIN,
+                       "css-name", "droptarget",
+                       NULL);
+}

src/utils/gcal-gui-utils.h

@@ -27,4 +27,6 @@ G_BEGIN_DECLS
 
 GtkWidget* gcal_create_selection_widget (void);
 
+GtkWidget* gcal_create_drop_target_widget (void);
+
 G_END_DECLS